What is a planet?
A planet is a celestial body that orbits a star, is spherical due to its own gravity, and has cleared its orbital neighborhood of other debris. According to the International Astronomical Union, an object must meet all three criteria to be classified as a planet. This definition separates the eight major planets in our solar system from dwarf planets like Pluto, which share their orbits with numerous other icy bodies in the Kuiper Belt.
What is a moon?
A moon, or natural satellite, is an astronomical body that orbits a planet, dwarf planet, or minor planet rather than a star.
Moons come in various sizes and compositions, ranging from jagged, captured asteroids to massive, geologically active spheres like Jupiter's Ganymede. They are formed either from the disk of gas and dust surrounding a young planet or from the debris of giant cosmic collisions, held in place by their parent body's gravity.
What is a nebula?
A nebula is an immense, interstellar cloud of dust, hydrogen, helium, and other ionized gases scattered across space. Often referred to as "cosmic nurseries," nebulae are the birthplaces of stars and planetary systems, where gravity pulls gas and dust together until they collapse to form new suns.
Conversely, some nebulae are created at the end of a star's life cycle during dramatic supernova explosions, dispersing enriched elements back into the universe.
What is gravity?
Gravity is the invisible, fundamental force of attraction that exists between all matter in the universe. On Earth, gravity is what keeps our feet on the ground, holds the atmosphere in place, and causes dropped objects to fall. On a cosmic scale, it governs the motion of planets, keeping Earth in orbit around the Sun.
What is the relationship between mass and gravity?
The relationship between mass and gravity is directly proportional: as the mass of an object increases, its gravitational pull becomes stronger. Every object with mass exerts gravity, but the effect is only noticeable with massive bodies like planets.
For example, because Earth has significantly more mass than the Moon, its gravitational pull is about six times stronger. This fundamental link dictates how planets hold onto their moons and how stars keep solar systems together.
What is the relationship between distance and gravity?
The relationship between distance and gravity is inversely proportional and follows the inverse-square law: as the distance between two objects increases, the gravitational force between them rapidly decreases.
Specifically, if you double the distance between two objects, their gravitational attraction drops to one-quarter of its original strength. This means that while gravity technically reaches infinitely across space, its power fades drastically over long distances, allowing local objects—like our Moon—to dominate Earth's tides.
What is accretion?
Accretion is the cosmic process by which gravity pulls together dust, gas, and rocky debris to form larger celestial bodies. In a young solar system, microscopic grains within a swirling protoplanetary disk collide and stick together via electrostatic forces.
As these clumps grow into pebbles, boulders, and eventually kilometers-wide planetesimals, their gravitational pull increases. This allows them to sweep up surrounding material at an accelerated rate, eventually forming the cores of fully realized moons and planets.
What is the Total Mass Index?
The Total Mass Index (TMI) is a metrics-driven calculation used in educational simulations to evaluate the physical development of celestial bodies or structures. By tracking the cumulative mass gathered during processes like planetary accretion, the index determines when a growing planetesimal has acquired enough matter to trigger gravitational differentiation or atmospheric retention.
It provides students with a clear, quantifiable data point to analyze how mass thresholds dictate a world's final classification, structure, and cosmic evolution.
What is the critical mass?
In planetary formation, critical mass refers to the threshold where a growing rocky core becomes massive enough to trigger runaway gas accretion.
Once a protoplanet reaches this tipping point—typically around 10 Earth masses—its gravitational pull becomes strong enough to rapidly draw in light hydrogen and helium gases from the surrounding nebula. This swift transformation allows a dense, rocky core to balloon into a massive gas giant like Jupiter or Saturn.
What is density?
Density is a fundamental physical property that measures how much mass is contained within a given volume of a substance. It is calculated using the formula:
Density = mass/volume
Essentially, it describes how tightly packed the atoms or molecules are within an object. Substances with high density have closely packed particles and feel heavy for their size, while low-density substances have loosely arranged particles and are lighter.
In this module, you will investigate how the raw materials of a nebula transform into structured planets and moons. Your challenge is to decode the laws of gravity, thermodynamics, and motion to map the birth of these worlds.
